The International Electrotechnical Commission, in standard IEC 60950-1, limits the current that can be drawn from a battery having less than 30 volts open circuit voltage to 8 amperes for battery powered devices. Accordingly, this standard applies to virtually all portable two-way radio devices, many of which operate at either a 3.6 volts or a 7.2 volts nominal operating voltage. These nominal voltage levels are achieved using one equivalent lithium ion battery cell for the 3.6 V nominal output voltage, or the equivalent of two lithium ion cells connected in series to achieve the 7.2 V nominal output. Since the current output of the battery is limited to 8 A, or less, by an internal safety circuit, more power is available from a battery having a higher nominal voltage than one having a lower nominal voltage. However, a 7.2V battery with higher output power than that of the 3.6V battery will have both weight and volume being two to four times that of the 3.6V battery for similar battery cell capacity. In a practical implementation of a 7.2V battery it would need to have four times the volume and weight of the equivalent 3.6V battery to enable it to source twice the power, for equal power dissipation, cell stress due to load current etc.
Some segments of the portable device market, including portions of the portable two-way radio device market, prefer low weight and low (spatial) volume devices, which tends to favor using a smaller battery in a single cell topology having a lower nominal voltage. Given a battery current limit, the use of a lower voltage battery presents a problem with regard to audio fidelity because the power available to the portable two-way radio device is less than that available from a substantially larger battery having a higher nominal voltage under the same battery current limit. When a portable two-way radio device receives an audio signal, the audio signal is amplified and played over a loudspeaker so the user of the portable two-way radio device can hear the audio signal without having to, for example, hold the speaker up to the user's ear as when using a telephone. Furthermore, given that the user of the portable two-way radio device can be located in a noisy environment, the audio power needs to be sufficiently loud and distortion-free for the user to understand received audio signals under such conditions.
An audio power amplifier is used to amplify received audio signals and drive a speaker with the amplified audio signal. A peak instantaneous power of, for example, 12 Watts is needed to provide the desired distortionless intelligibility and output power to meet certain industry standards. However, given the limitation on current that can be drawn from the battery under IEC 60950-1, and given the current budget needed to concurrently operate all portions of the portable two-way radio device (e.g. RF transmitter, RF receiver, controller, audio power amplifier etc.) without exceeding the battery current limit, and in view of the losses due to inefficiency, it is not possible to draw the necessary instantaneous current directly from the battery to support instantaneous 12 W peak audio power. As a result, using conventional audio power amplification that relies on the combination of audio power amplifier supply voltage, or maximum peak output voltage of the audio power amplifier, and loudspeaker impedance to limit the peak current drawn from the battery to the budgeted value, peak clipping will occur, causing significant distortion and reducing intelligibility.
Accordingly, there is a need for a method and apparatus for amplifying received audio signals in a portable two-way radio device under current budgeted conditions in a way that does not exceed a budgeted current limit and which provides sufficient peak audio power to allow a user of the portable two-way radio device to hear and understand the received audio in noisy environments.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.